2Digital Design Experiment: D1 - 02ExpressLandCardExpressland buses accept payment only in the form of contactless smart&cards, ExpressLandCards. The card can store a maximum VALUEof S$15, which is shown in binary using LEDs. When the user boards the bus, he/she taps the card against the reader (which is simulated using a START/STOPpush-button. From then on, for each fare stage, $1 is deducted. One bus trip consists of a total of 5 stages, and the stages get incremented once every 3 clock cycles. Lack of sufficient value (a minimum of $5) in the card is shown using an INSUFFICIENT_VALUELED for one clock and boarding is denied. The user taps the card at the exit (which again is input through the START/STOPpush-button), when he is alighting, and no further fare is deducted. The bus starts a fresh trip immediately following the last stage of the current trip. At the end of a trip, if the user has not tapped the card at the exit, it is assumed that he/she has alighted at the last fare stage. On pressing a TOP_UPpush-button, the card value can be reset to S$15. The card cannot be topped up during a ride. Optional: A “LOW_VALUE” LED lights up whenever the balance falls to $7 or less. The Digital Electronics laboratory has stocks of popular TTL integrated circuit components that are listed below. If you plan to use a TTL IC which is not in the list, you should check the availability with the lab staff. You can collect the ICs you need for your project from the labafteryour design has been verified and approved. You will be given a custompackage containing a breadboard, a 7805 voltage regulator IC, push-button switches, 4-bit dip switches and LEDs tailored to the needs of the project assigned to you. Description Part Number Hex inverters 7404 Quad 2-input NAND gates 7400 Quad 2-input NOR gates 7402 Quad 2-input AND gates 7408 Dual 4-input NAND gates 7420 Dual 4-input AND gates 7421 Quad 2-input OR gates 7432 Quad 2-input XOR gates 7486 4 bit magnitude comparator, 7485 3 to 8 Line decoder / demultiplexer 74138 Dual 2 to 4 line decoder / demultiplexer 74139 Quad 2 to 1 line multiplexer 74157 Synchronous 4-bit up/down counter with dual clock Synchronous 4-bit up/down counter with mode control Synchronous mod-16 counter with synchronous clear and preset74193 74191 74161/74163 Dual D-type flip flop 7474 4 bit universal bi-directional shift register 74194 Binary adder 74283

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